When a fault occurs in a power transmission network, travelling waves arise which move along the line. It is known to use the direction of movement of these travelling waves as a measuring point to determine the direction to the location of the fault.
U.S. Pat. No. 3,878,460 discloses how to utilize the fact that in those travelling waves, which from a fault point move in towards the measuring point of the protection device, the current and voltage waves have different signs. If the voltage is designated "u", the current "i", and the wave impedance Z.sub.0, the equation u=-Z.sub.0 .multidot.i is obtained in the case of a fault lying ahead. If the fault is located behind the measuring point, the current and voltage waves have the same signs, whereby the equation u=Z.sub.0 .multidot.i is obtained for a external fault lying behind. To obtain a measure of the travelling waves, the fundamental frequency parts of the current and voltage signals are here filtered away.
A directional wave detector according to U.S. Pat. No. 4,351,011 (=SE 7803868-4) is using an alternative in which, instead of treating the voltage and current waves separately, the product of voltage and current is formed, that is, the power or its integral, that is, the energy. The direction to a fault can then be determined by the sign of the instantaneous power or energy change. For an internal fault or a fault lying ahead, a negative sign of the power or energy change is then obtained, and, in a corresponding way, a positive sign is obtained for an external fault or a fault lying behind.
Another way of obtaining directional detection of faults on a power line located between two stations P and Q is clear from U.S. Pat. No. 4,731,689. The method is based on the fact that with access to data for a faultless power line, a wave guide model of the power line can be determined. With the aid of the wave model and measured currents and voltages in the stations, the voltage distribution along the power line can be calculated. The direction to a fault is determined by studying locally in a station changes in calculated voltages in the two stations. If a fault occurs between the stations, the voltage change in station Q between the voltage prior to the fault and after the fault may be calculated, with the wave model, to be .vertline..DELTA.Uq.vertline. and the corresponding voltage change in station P may be calculated to be .vertline..DELTA.Up.vertline., and if .vertline..DELTA.Uq.vertline.-.vertline..DELTA.Up.vertline.&gt;0 this means that there is a fault on the line side of station P, that is, a fault lying ahead.
To be able to carry out the directional determination according to the invention, access is also needed to the corresponding change values or .DELTA.-values for both voltage and current. Several alternative methods are available for obtaining these values. One such method is described in the above-mentioned U.S. Pat. No. 4,731,689 in the following way: .DELTA.-values ". . . are obtained by adding the (voltage) values of two consecutive half-periods". Another alternative is to take the difference between the value in question and a value one period earlier. The generation of these .DELTA.-values will take placed, according to the description, in a so-called .DELTA.-filter.
In addition, it is necessary to have access to some model of the line which describes its properties. Also in this case there are several more or,less complete models which take the line data into account. One such model may, for example, be: EQU UM=R.multidot.I+L.multidot.I'+Rn.multidot.In+Ln.multidot.In'
Here, UM designates a model phase voltage, R designates the resistance of the line, I is the phase current, L is the inductance of the line, I' is the derivative of the phase current, Rn is the resistance of the ground loop, In is the ground current, Ln is the inductance of the ground loop, and In' is the derivative of the ground current. The formation of the phase voltages of the line model will be carried out, according to the following description, in a so-called "Line model".